An example of a conventional monolithic electronic circuit breaker is described in U.S. Pat. No. 5,581,433. The circuit breaker described therein includes a power MOSFET ("FET") switch that is connected between an input terminal and an output terminal. The power FET is controlled by a maximum current circuit and an over-current circuit. In the maximum current circuit, a linear amplifier compares a signal indicative of the current flowing through the power FET (i.e., the output current of the circuit breaker) with a signal indicative of a maximum current level. The output of the amplifier is coupled to the gate of the power FET in order to open the MOSFET switch. Similarly, the over-current circuit has a comparator that compares the output current with a fault current level threshold, and the output of the comparator is supplied to a timing control circuit. The output of the timing control circuit also controls the state of the switch. The maximum current level and fault current level may be set using external signals or fixed internal reference voltages. Additionally, the circuit breaker of this reference includes such conventional features as a thermal shutdown circuit, a reverse voltage shutdown circuit, and an externally-controlled shutdown control line.
In a typical application, the electronic circuit breaker is connected between an electronic device and a computer bus. When initially connected to the bus or when power is applied, the electronic device immediately begins to draw current from the bus to charge its internal capacitors. The circuit breaker controls and limits the current that the device draws from the bus. In particular, when the output current of the circuit breaker (i.e., the current drawn by the electronic device) is below the fault current level, the switch is closed and the amplifier drives the power FET with a boosted voltage so that the power FET is at maximum conductance. If the output current goes above the fault current level, the comparator signals the timing control circuit and the timing control circuit begins timing the fault condition by charging a capacitor.
If the fault condition persists after the charging time for the capacitor, the switch is opened to turn off the power FET. As a result, current stops flowing through the circuit breaker to eliminate the fault condition. After the period lapses to discharge the capacitor to a second voltage level, the timing control circuit allows current to resume flowing through the circuit breaker transistor. If the fault current level is again exceeded, the timing control circuit begins to recharge the capacitor and the process described above is repeated. Further, if the output current ever exceeds a higher, maximum current level, the current amplifier adjusts the voltage used to drive the power FET so that current is restricted to the maximum current level. In this manner, the circuit breaker permits current above the fault current level and up to the maximum current level until the fault time is exceeded. When the fault time is exceeded, current is discontinued for a predetermined time period and is then restarted. This cycle of temporarily interrupting current is repeated until the fault condition is removed.
Thus, such a conventional circuit breaker is continuously reset regardless of the number of times a fault condition causes the circuit breaker to interrupt the current. However, if a defective or inappropriate electronic device is connected to the bus or a short exists, an over-current condition may exist indefinitely. In other words, the fault condition will persist regardless of the number of times the circuit breaker is reset. In such a situation, the conventional circuit breaker would allow current to flow up to the maximum current level until the fault time is exceeded each time the circuit breaker is reset. This causes unnecessary power consumption and may allow the computer or electronic device to be damaged.
Additionally, the power FET is immediately returned to maximum conductance when the conventional circuit breaker is reset. Thus, a large current can immediately begin flowing through the circuit breaker during the power-up of the electronic device. Such large currents can cause an over-current condition that arbitrarily causes the circuit breaker to trip. Such large start-up currents can also damage the attached device. Besides unnecessarily tripping the breaker, such a situation causes the circuit breaker to incorrectly signal an error, and may increase the time required to power-up a device.